At a front end section of a communication terminal such as a mobile phone or the like, use is made of a duplexer that separates the transmission and reception frequencies.
A duplexer has an antenna terminal, a transmitting terminal, and a receiving terminal. A transmitting filter is arranged between the antenna terminal and the transmitting terminal, while a receiving filter is arranged between the antenna terminal and the receiving terminal. In a communication terminal, a transmitting circuit and a receiving circuit are arranged after the duplexer. The duplexer has a function of branching a transmission signal from the transmitting circuit to the antenna terminal and branching a reception signal received at the antenna terminal to the receiving circuit.
In such a duplexer, the transmitting filter and the receiving filter are matched in order to prevent the transmission signal from flowing into the receiving circuit or to prevent the reception signal from flowing into the transmitting circuit.
However, depending on the individual filter design, even in a state where the best match is obtained, for example, sometimes the impedance to the receiving filter becomes slightly lower than the impedance from the transmitting filter to the antenna or sometimes the impedance to the transmitting filter becomes slightly lower than the impedance from the antenna to the receiving filter. In this case, the signal which inherently should pass from the transmitting filter to the antenna will flow to the receiving filter, or the signal which should have been input from the antenna to the receiving filter will flow to the transmitting filter, so the isolation characteristic is deteriorated.
The transmitting filter and receiving filter used in a duplexer are frequently configured by surface acoustic wave filters with IDT electrodes formed on piezoelectric substrates. In a conventional duplexer, the isolation characteristic was improved by adjusting the impedance of the each filter by changing the distances between electrode fingers of the IDT electrodes or the numbers of electrode fingers (see, for example, Patent Literature 1).
In this regard, as required specifications demanded from a duplexer, other than the isolation characteristic, the attenuation characteristic has become important as well. This is because deterioration of the quality of speech of a mobile phone and other problems occur if either characteristic of the isolation characteristic and attenuation characteristic is poor.
With the conventional adjustment methods, however, the adjustable range was limited, so it was difficult to satisfy required specifications of the isolation characteristic and attenuation characteristic together.
The present invention was made in order to solve the above problems and provides a duplexer capable of improving the isolation characteristic and attenuation characteristic and a communication device using the same.
Patent Literature 1; Japanese Patent Publication (A) No. 5-183380